1. Field of the Invention
The present invention relates to an apparatus, a signaling method, and a tangible machine-readable medium for callback handover procedure for network composed of outdoor base station (BS), outdoor relay, indoor base station (Femto BS) and indoor relay; more specifically, relates to an apparatus, a signaling method, and a tangible machine-readable medium for callback handover procedure for indoor and outdoor base stations and relays under the IEEE 802.16m and LTE/LTE-A standards.
2. Descriptions of the Related Art
Although the IEEE 802.16 and LTE/LTE-A standards already provide greater bandwidths, lower building cost, better service quality and expansibility, there still are some defects of coverage and signal quality of the IEEE 802.16 and LTE/LTE-A standards. Therefore, the IEEE 802.16/LTE/LTE-A standard working groups have included the indoor femto base stations (BSs). The major difference between an indoor femto BS and an outdoor BS is the media type of backhaul link. Femto BS uses the wired link, such as ADSL and Cable link, as the backhaul link.
Please refer to FIG. 1, which illustrates the cellular network topology, which comprises an mobile station (MS), an outdoor base station (BS), an indoor femto BS, a backbone network and a network mobility controller, wherein the MS is one type of subscriber station (SS) which has mobility. The MS intends to access the backbone network via the outdoor BS and/or indoor femto BS. The considered scenario is that the type of backhaul link of femto BS is wired link and the femto BS can communicate with the other BSs and network mobility controller via backbone network. It can be understood that, a femto BS will disable its air interface unless the backhaul connection with the backbone network is fully established.
The handover procedure adopted in the IEEE 802.16e/LTE/LTE-A standards, is a technique, which allows an MS to change the associated BS when the channel condition is unsatisfactory. That is, such conventional handover procedure is designed to force an MS to disassociate with its serving BS according to the metrics of signal quality, network load, and so on. Since the backhaul link of femto BS may disconnect/reconnect from/to backbone network sometimes, the femto BS will command all the associated MSs to handover to the other BS when it disconnects from backbone network. There are two main defects in conventional handover mechanism. First, there is no out-bound handover reason provided to the MS. The MS then cannot distinguish that the handover signal is triggered by bad channel quality, disconnection of backhaul link or the other reasons. So the MS may not record the information of the femto BS, which issues the out-bound handover signal. Second, there is no in-bound handover reason provided to the MS, the MS cannot distinguish that handover signal is triggered by bad channel quality, recovery of backhaul link or the other reasons. The MS may take a long time to handover to the previously serving femto BS which provides a good channel quality. Accordingly, how to improve the performance of the out-bound and in-bound handover in femto-networks is still an objective for the industry to endeavor.
Since the backhaul link of femto BS may disconnect/reconnect from/to backbone network sometimes, the method for fast handling the associated MSs to leave from and return to the femto BS should be supported in order to continue the services and optimize the network performance. As the serving femto BS loses the backhaul connection, the femto BS shall disable its downlink air interface transmitter until the backhaul connection with the backbone network is fully recovered. Accordingly, a femto BS should be able to command all the associated MSs to perform out-bound and in-bound handover procedures in order to continue services. The out-bound handover signal includes the reason code, which is used to notify the MSs about the discontinuous services of serving femto BS and thereby the MSs record the information of serving femto BS and then handover to the other BSs. As the femto BS reconnects to the backbone network, the femto BS and/or the network mobility controller issues in-bound callback handover signal with reason code to neighboring femto BSs and outdoor BS to call back those MSs, who left from the previously serving femto BS. Upon receiving the in-bound callback handover signal with specific reason code from either femto BS or outdoor BS, the MS will try to conduct optimized handover to the previously serving femto BS if the signal quality is better than the current connection. In a word, the primary objective of this invention is to provide a signaling method for bi-directional handover with reason code in a femto-network.